The invention relates to a method of adapting image information to the perceptive capacity of the human eye.
Color is a visible sensation, which is triggered by electromagnetic radiation and is conveyed by the eye. Only in the brain does the conversion take place between the energy of the color stimulus coming from outside and the actual color sensation. Color is therefore not an objective physical property of a body or a light source but a subjective perception. In this case, the perception of lightness steps and color steps depends to a great extend on the respective lightness and color or color saturation. For example, it is possible that different color or lightness steps, which may be detected reliably in terms of the frequency and amplitude of the electromagnetic radiation in the visible range, are to some extent not perceived at all, or are perceived only weakly or in distorted form.
In order to describe colors, color spaces based on different principles are used. For example, there is the RGB color space which describes an additive color system, the HSB color space based on the latter, the CMYK color space, which describes a subtractive color system, or the CIE standard color space which describes all the colors which can be perceived by the human eye. The common factor in all these color spaces is that the color difference between colors which in each case are at the same distance from one another within the color spaces are perceived differently in physiological terms, for the reason mentioned above.
By contrast, there are various models for transforming color spaces into perceived color spaces, in which the color difference corresponds to the physiologically perceived color difference. For example, the CIE-L*a*b* system is based on a mathematical transformation of the CIE standard color space.
The subjectively different perception of color differences, in particular in the visual evaluation of image information, results in a considerable problem. For example, in the original of an X-ray image, there is contained a large amount of measurable image information, whose display in various closely adjacent gray steps cannot be distinguished or can barely be distinguished by the human eye. This information therefore cannot be assessed visually at all, or only poorly.
For the improved evaluation of radiological image data, in particular, DE 4437467 C1 proposes emphasizing linear structures more clearly, by individual image points being emphasized by the image signal values from these image points being amplified as a function of the image signal values from adjacent image points. If the contrast between mutually spaced image points is high, this contrast is enhanced. If it is low, no enhancement occurs. The method is therefore not suitable to make it possible for image points located close beside one another and having very similar colorimetric values to be distinguished better from one another. A further disadvantage is that the image information is distorted and, as a result, falsified. Finally, the mathematical method which necessarily has to be used is complicated.